This (very, very old) applet lets you select one or more overtones of the A string
(second to lowest) on the guitar and hear them.

Right now, all the sounds have the same wave amplitude, so it gets louder the more overtones you add. Adjust your sound device's wave-generator volume accordingly.
Real overtones,
of course, get weaker as you go higher in frequency.
I may add a volume control to each sound so you can synthesize your own
overtone spectrum.

One interesting thing you can do with this applet is make a
sound with the fundamental missing. The fascinating
thing is that you can still hear the fundamental, because when
two neighboring harmonics mix, they produce a fundamental in
what is called beats. Try selecting only 220, only 330 and then
220 and 330 Hz together, for example.